The present invention relates to an insulated gate semiconductor device used as a switching device.
Conventionally, MOSFET and Insulation Gate Type Bipolar Transistor (hereinafter referred to as IGBT ) are well known as vertical type power semiconductor devices which have advantages in high speed switching characteristics, high input impedance and low input loss. In order to reduce the resistance of the junction field effect transistor (hereinafter referred to as JFET) immanent in above individual semiconductor devices and establish low-loss, what are used for both transistors are semiconductor devices having a trench type insulated gate structure having a gate 14 in the concave portion 29 as shown in FIGS. 11 and 12.
In such conventional semiconductor devices having a trench type insulated gate structure as shown in FIGS. 11 and 12, in case that the carrier density in the nxe2x88x92 conduction type drift layer 2 as a semiconductor substrate having the first conduction type (n) is larger, the formation of a channel is blocked by making the gate electric potential smaller than the source electric potential (equivalent to the emitter electric potential in FIG. 12). In this case, if a high voltage having straight polarity is applied between the drain and the source (equivalently between the collector and the emitter in FIG. 12), a depletion layer is developed at the junction between the nxe2x88x92 conduction type drift layer 2 and the p conduction type body layer 4 as a semiconductor layer formed on a surface portionially or wholly of the semiconductor substrate with the first conduction type, having the second conduction type (p) opposite to the first conduction type (n) and forming a junction with the nxe2x88x92 conduction type drift layer 2.
However, as the carrier density of the nxe2x88x92 conduction type drift layer 2 is high below the gate 14 and the electric conductivity is larger, the resistance of the layer becomes smaller. As a result, the voltage applied at the nxe2x88x92 conduction type drift layer 2 becomes smaller, and thus, a high voltage is applied at the bottom portion of the insulation layer 9 formed in the inner surface of the concave portion 29. Owing to this characteristic, as the electric field strength at the bottom portion in the insulation layer 9 at the bottom portion of the trench type insulated gate becomes higher, the withstand voltage of the insulator layer 9 is bound at most up to the level at the insulation breakdown, and hence, the high-voltage adaptability of the device can not be easily established. In addition, as the higher electric field strength in the insulator layer 9 leads to the deterioration of the insulator layer 9, the establishment of higher reliability of devices is difficult.
An object of the present invention is to provide a insulated gate semiconductor device having high-voltage adaptability and high reliability which allows to relax the electric field strength at the bottom portion of the trench type insulated gate.
In order to solve the above described problems, in the present invention, a first semiconductor region having a second conduction type formed in the semiconductor substrate, that is, a semiconductor region for the relaxation of the electric field is defined at the bottom portion of the trench type insulated gate.
A characteristic of the present invention is based on a insulated gate semiconductor device having a semiconductor substrate having a first conduction type, a semiconductor layer of the second conduction type opposite to the first conduction type formed on the semiconductor substrate and forming a junction with the semiconductor substrate, at least one concave portion penetrating the semiconductor layer and reaching and cutting portionially the semiconductor substrate, a first semiconductor region of the second conduction type formed in the semiconductor substrate at the bottom portion of the concave portion, an insulator layer formed on the internal surface of the concave portion, a gate insulated by the insulator layer with the substrate and the semiconductor layer and at least portionially formed in the concave portion, a second semiconductor layer of the first conduction type formed at the surface of the semiconductor layer of the second conduction type with a designated depth at the peripheral area of the gate surrounded by the insulator layer in the semiconductor layer, a first electrode formed on the semiconductor layer of the second conduction type and the semiconductor region and defined to be electrically conduction to the semiconductor layer of the second conduction type and the semiconductor region, and a second electrode defined at another portion of the semiconductor substrate.
According to the present invention, by means of forming a semiconductor region below the trench type insulated gate of the trench type insulated gate semiconductor device for the relaxation of the electric field, in case of applying a voltage with a straight polarity between the drain and the source (or between the collector and the emitter), if the drift layer is a first conduction type in FIGS. 1 to 10, a depleted layer develops in the body layer of the second conduction type and the drift layer of the first conduction type. On the other hand, at the bottom of the trench type insulated gate electrode, a depleted layer extends from the junction between the semiconductor layer for the relaxation of the electric field and the drift layer of the first conduction type in responsive to the voltage between the drain and the source (or between the collector and the emitter), and the almost all of the applied voltage is supported by the semiconductor region for electric field relaxation and the drift layer of the first conduction type. As a result, the voltage assigned to the bottom portion of the insulator layer of the gate becomes smaller and the electric field strength of the insulator layer is relaxed, and consequently, higher high-voltage adaptability or higher reliability of the semiconductor device can be achieved.
The word xe2x80x9ctrenchxe2x80x9d used in the present invention relates to a concept including various types of holes and concave portions other than channels.
Another characteristic of the present invention is that the thickness of the insulator layer of the bottom portion of the trench type insulated gate is made to be much larger than the thickness of the lateral insulator layer. Owing to this structure, higher high-voltage adaptability or higher reliability of the semiconductor device can be achieved. In this case, by forming a semiconductor region for the electric field relaxation much higher high-voltage adaptability or higher reliability of the semiconductor device can be achieved.